Clean electric power generation process

ABSTRACT

A process is disclosed for the production of power, particularly in the form of electricity from a carbonaceous fuel which comprises partially oxidizing the fuel with oxygen or an oxygen-containing gas to yield a gas stream containing carbon monoxide at supra-atmospheric pressure, expanding the said gas stream to produce power, and substantially completely combusting at least a major portion of the expanded stream with additional oxygen or an oxygen-containing gas to produce additional power, characterized in that, prior to expansion, the said gas stream is subjected to a carbon monoxide shift reaction whereby at least some of the carbon monoxide therein is converted into carbon dioxide and hydrogen, and in that at least some of the heat of such shift reaction is used to reheat the gas stream prior to expansion. 
     By using the process, greatly reduced emissions of nitrogen oxides can be achieved which will help reduce acid rain.

This application is a division of U.S. patent application Ser. No.07/309,966 filed Feb. 13, 1989, now U.S. Pat. No. 4,881,366, which is acontinuation of U.S. patent application Ser. No. 07/090,094 filed Aug.27, 1987, now abandoned.

This invention relates to a process for the production of power,particularly electric power, from a carbonaceous fuel using partialoxidation of that fuel.

The present invention provides a process for the production of powerfrom a carbonaceous fuel which comprises partially oxidising the fuelwith oxygen or an oxygen-containing gas to yield a gas stream containingcarbon monoxide at supra-atmospheric pressure, expanding the said gasstream to produce power, and substantially completely combusting atleast a major portion of the expanded stream with additional oxygen oran oxygen-containing gas to produce additional power, characterised inthat, prior to expansion, the said gas stream is subjected to a carbonmonoxide shift reaction whereby at least some of the carbon monoxidetherein is converted into carbon dioxide and hydrogen, and in that atleast some of the heat of such shift reaction is used to preheat the gasstream prior to expansion.

Optionally: the gas stream from the partial oxidation reactor isquenched with water prior to the shift reaction step, at least some ofthe steam required for the shift reaction step being derived fromevaporation of the quench water: the temperature of the gas stream afterthe shift reaction is increased prior to expansion: the reacted andexpanded stream is processed to remove sulphur compounds beforeutilising at least a part of the stream as fuel to produce electricity.

Several power-producing processes are known which are based on thepartial oxidation of a carbonaceous fuel and in which there is mentionof reacting carbon monoxide and steam to produce carbon dioxide andhydrogen. This reaction, often known as the watergas shift reaction, theshift reaction, shift or shifting, is well known in synthesis gasproduction where an increase in the amount of hydrogen relative tocarbon monoxide is desirable, even when the concomitant increase incarbon dioxide is not. EP-A-9524 discloses such a process but in thiscase carbon dioxide is an impurity which must be removed.

In U.S. Pat. No. 4,074,981 (Column 2 line 29 et. seq.) the various usesof gas produced by their invention are described. For synthesis gas,hydrogen and carbon monoxide are maximised, implying that carbon dioxideis minimised. For use as a reducing gas, carbon dioxide is stated to beminimised. For use as a fuel gas having a high heating value, hydrogen,carbon monoxide and methane are maximised, again implying carbon dioxideis minimised.

U.S. Pat. No. 4,202,167 discloses a process in which the carbon dioxidegenerated by the shift reaction is used, but there is no recognition ofthe power recovery that is possible by expanding the raw gas after shiftand before their final combustion.

Swiss Patent 250478 discloses the use of an expander between a gasgenerator and the combustion chamber of a gas turbine.

U.S. Pat. No. 3,720,625 discloses a process for the production ofhydrogen and/or ammonia, and gives the conventional use of the shiftreaction.

In this present invention, the carbonaceous fuel may be gasified bypartial oxidation at pressure by a number of methods well known to thoseversed in the art (e.g. U.S. Pat. No. 2,992,906). These methods normallyinvolve gasifying the carbonaceous fuel with an oxygen-containing gase.g. air, or preferably with a substantially pure oxygen stream.Temperatures of the order of 1000° C. to 1600° C. are reached. Thepressure of this partial oxidation may be in the range of 15-250 barsbut is more likely to be in the range of 40-150 bars. Examples ofsuitable carbonaceous fuels are crude oil, coal, natural gas, naphtha,and heavy fuel oil. Lignite may also be used.

Preferably, any particulate matter contained in the hot gas stream fromthe partial oxidation reactor is removed prior to reacting the carbonmonoxide to avoid fouling the catalyst normally used to promote theshift reaction, although a guard bed may also be used. Any conventionalmethod can be used e.g. electrostatic precipitation, water washing,cyclones, filters, but preferably particulate matter is removed byquenching with water in a manner to wash it out. By using a quench afterthe partial oxidation reactor, greater flexibility is obtained regardingoperating pressure of the partial oxidation reactor and, moreimportantly, the type of carbonaceous fuel, particularly coal, used.

The gases from the partial oxidation reactor may be cooled in a boilerand/or be quenched prior to expansion.

Since many fuels contain sulphur, a sulphur removal step is generallyrequired. This step takes places prior to final combustion.

It is possible to remove sulphur compounds prior to the shift reactorand/or expander. However, present day sulphur removal systems work atrelatively low temperatures. These temperatures mean that most of thesteam, which is present after quenching the hot gases, would condenseout. In order to subsequently react the carbon monoxide with steam,steam would have to be added.

In removing the sulphur compounds some carbon dioxide may also beremoved, but the intent is to retain as much of the carbon dioxideinitially present as is economic.

This is an important part of this present invention. The carbon dioxidecontained in the gas reduces the NO_(x) formed when the gas is burnt asfuel. Also, the expansion of the carbon dioxide produces power both whenthe gas is expanded after shift and when the gas is finally burnt.

Water or steam necessary for the shift reaction may arise by theaddition of water or steam to the partial oxidation reactor, and/or byreaction in the partial oxidation step, and/or in a quench stepfollowing the partial oxidation step and/or by the direct addition ofwater or steam.

In the preferred embodiment some of the water or steam required to reactwith the carbon monoxide, and to remain at the end of the reaction togive the desired shift reaction equilibrium, is added as a result ofevaporating quench water into the very hot gases leaving the partialoxidation reactor. This has the added advantage of cooling the hot gasesfrom the partial oxidation reactor so as to make them more easilymanageable. As regards cooling, partial or complete evaporation of thequench water may take place. However, for reasons given above, partialevaporation is preferred.

Generally, shift reaction catalysts require water to maintain theiractivities. Typically, the molar water to dry gas ratio on entry to thecatalyst is within the range of 0.3 to 1.7. Preferably the ratio is 0.5to 1.2. The gases are then allowed to react adiabatically. With moderncatalysts, the effluent gas will closely approach the shift reactionequilibrium.

A shift catalyst may comprise iron oxide mixed with Cr oxide andpromoted by 1 to 15 wt. % of an oxide of another metal, such as K, Th,U, Be, or Sb. Reaction occurs at 260° C. to 565° C. (500° C. to 1050°F.).

The expansion takes place after the shift reaction. Preferably the gasesare expanded immediately after this reaction, since the heat releasedduring the reaction raises the temperature of the gas stream and therebymakes the expansion more efficient. However, additional heating of thisstream between shift and expansion steps can be effected e.g. by furtherpartially oxidising the gases or heating in the exhaust convectionheating zone of the downstream gas turbines. Alternately the heat of theshift reaction may be used indirectly to heat the gases prior toexpansion.

The expanded gas stream may be used to preheat the feed to the shiftreactor.

Some advantage would be gained even if the gas were cooled between shiftand expansion (notwithstanding that the preferred embodiment is not tocool at all). Thus, this invention is also applicable when the shiftedgas is cooled to any temperature down to 204° C. (400° F.) and morepreferably, not below 330° C., before expansion.

A number of advantages arise from the use of the combination of shiftreaction and expansion in a power-generating process:

(a) some of the carbonyl sulphide (COS) present is simultaneouslyreacted to hydrogen sulphide, in which form the sulphur may more easilybe removed;

(b) the flame temperature of the gas is significantly reduced giving thevery important advantage of reducing the amount of oxides of nitrogen(NO_(x)) formed, which should help to reduce acid rain;

(c) (b) above means that significantly less or even no water or steamhas to be added to the stream to be burnt to lower its flame temperaturein order to reduce NO_(x) formation, which in turn can improve the lifeof the turbine blades; and

(d) the exothermic shift reaction can usefully be used to preheat thestream prior to it being expanded.

The production of electricity can take place with the use of a gasturbine and/or may involve the use of a steam system. Steam may beraised at any suitable stage in the present process, for example in theexhaust of any gas turbines which are used.

In order to further reduce the amount of nitrogen oxides (NO_(x))produced when the gas is finally burnt, the fuel gas may be saturatedwith water. Furthermore it may be heated before and/or after suchsaturation, or it may be heated without saturation (or the addition ofwater or steam) so as to present a hot fuel gas to the burners.

One embodiment of the present invention will now be described by way ofexample, with reference to FIG. 1 (for convenience divided into parts 1Aand 1B) and Table 1. In this example, the feedstock is an emulsion ofheavy crude oil and water, having a sulphur content of about 2-3%sulphur by weight.

For practical sizing of equipment, the actual plant would consist of twogas generation lines in parallel feeding three gas turbine lines inparallel, feeding into one conzone. However, for simplicity, thefollowing example is based upon one gas generation line all the waythrough the plant.

The feedstock emulsion is reacted with 99.5% pure oxygen at a pressureof 70 bar in a partial oxidation unit. The resulting mixture of gases isquenched using an excess of water, i.e. not all of the water evaporates,down to the saturation condition at a pressure of 60 bar and at 244° C.The partial oxidation unit and quench is shown as item (10) on FIG. 1.This quench step is therefore a gas washing step in addition to a gascooling step.

The gas produced after quenching is as stream 1 of Table 1. The gas iscooled to 232° C., by taking immediate advantage of its heat content toraise about 45 tonnes per hour of steam in boiler (11) which is fed withboiler feed water (BFW) at the relatively high pressure of 20 bars.Water condensed out of the cooled gas is removed in drum (12) leaving araw gas with a steam to dry gas ratio of 1.0. The removed water isrecycled by pump (20) back to the partial oxidation unit (10) as quenchwater. After preheating in heat exchanger (13), the gas (stream 2 ofTable 1) enters a catalytic shift reactor (14) at 330° C. The shiftreaction takes place adiabatically to produce a shifted gas (stream 3 ofTable 1) at 508° C. and about 58 bar. The shifted gas is immediately letdown through a hot gas expander (15) to about 28 bars and 396° C.,thereby generating 25MW of power.

The expanded gas is close to its dew point and is then cooled in heatexchangers (13) and (16) down to 200° C. The heat removed is used topreheat the feed to shift reactor (14) and to heat the product fuel gasin exchanger (16) prior to final combustion. After heat exchanger (13)the temperature is 295° C. The cooled shifted gas then enters anothercatalytic reactor (17) where COS (carbonyl sulphide) is reduced to a lowlevel to give a stream flow as stream 4 of Table 1. This step isdesirable because COS is much more difficult to remove by sulphurremoval processes than hydrogen sulphide, produced from it.

Further heat is recovered from the gas from reactor (17) by a waste heatboiler (18), in which about 65 tonnes per hour of 5 bar steam areraised, and condensate is removed at drum (19) at 160° C. Part of thiscondensate separated in drum (12) joins that produced at 232° C.upstream (separated at (19)) and is returned by the pump (20) to partialoxidation unit (10) for raw gas quenching. As there is still asubstantial amount of useful heat present in the gas leaving drum (19),some heat is utilised in an exchanger (21) to drive an absorptionrefrigeration unit (28) dropping the temperature from 160° C. to 145°C., and some is utilised in a reboiler (22) in a sulphur removal unit(27). After further condensate removal in knock-out drum (23), theremaining useful heat of the gas stream is used for general waterheating using heat exchanger (24). The stream is finally cooled to 40°C. using cooling water using heat exchanger (25), condensate beingremoved in drum (26).

The cooled gas (stream 5 of Table 1) is now at 26 bar and contains about32% carbon dioxide by volume, but is contaminated with about 1% ofsulphur compounds, largely H₂ S. The sulphur removal unit (27) should becapable of removing all sulphur down to less than about 50 ppm whileretaining most of the carbon dioxide in the gas stream. Severalselective processes are currently available, such as those known by thenames, Sulphurox, Selexol, Purisol or Alkazid. In some cases, theyrequire reboil heat for stripping the H₂ S from the solvent, togetherwith refrigeration to minimise the large liquor circulation rates whichare inherent in processes which use physical solvents. In FIG. 1B, asulphur removal unit (27) is shown which uses a physical solvent.Refrigeration is supplied by the ammonia absorption unit (28), for whichthe necessary heat energy was taken from the gas stream in exchanger(21). Similarly, the reboil heat for solvent stripping within thesulphur removal unit is provided by exchanger (22).

The sweetgas (stream 6 of Table 1) leaves the sulphur removal unit (27)at 40° C. and 25 bar, essentially free of sulphur. After splitting offsome of the gas for use as gas turbine column exhaust afterburn (stream11 of Table 1), the balance (stream 7 of Table 1) passes into a packedsaturator column (30) where it is contacted with a circulating stream ofwater at 150° C. The heat for this water is derived from heating coil(40) in a gas turbine (31, 32, 33) exhaust convection heating zone(conzone) (34A). Fuel gas leaves the column (30) saturated with steam at130° C. and is further heated to 280° C. by heat recovery from the rawshifted gas in exchanger (16).

The heated fuel gas for the gas turbine is fed to the combustion chamber(31) and is burnt with air from the compressor (32) which is driven bythe power turbine (33). The net power output drives an alternator (52)to produce electrical power. In this example the fuel gas fed to the gasturbine (stream 10 of Table 1) at 280° C. and 24 bars contains about 27%of carbon dioxide by volume (dry basis), 11% by volume of steam. Thisconstitutes a highly effective gas turbine fuel because of the presenceof 38% by volume inerts, mainly carbon dioxide, already at thecombustion chamber pressure. This reduces the air compressor load andhence increases the net amount of power available for electricitygeneration. For the described embodiment of this invention, the gasturbines will give a net power of 225MW.

The exhaust gases (stream 12 of Table 1) leave the gas turbine at about470° C. In order to make optimum use of the large amount of heat presentin these gases, the temperature is first raised to 575° C. in anafterburner (34) which uses as its fuel some of the sweetgas (stream 11of Table 1) taken off at an earlier stage. The effluent gas is thenpassed to the conzone (34A) where it passes over a series of heatrecovery coils used in various steam raising and superheating duties.The waste heat boiler coil (36) and its associated water heater heatingcoil (38) have the capacity to raise about 370 tonnes per hour of steamat 100 bars. This steam is superheated in coil (35) to 500° C. and thenpassed to the linked steam turbines (41-44). These operate between 100bars and condensing conditions at 0.03 bars with an intermediate reheatat 5 bars. The four turbines together generate about 157MW.

In addition, the 45 tonnes per hour of 20 bar steam raised immediatelyafter the quench, i.e. in boiler (11),is superheated in coil (37) of theconzone (34A) to 290° C. and is passed to the turbine (42). Theadditional 5 bar steam raised in waste heat boiler (18) is added to the5 bar steam and together they are reheated from about 160° C. up to 275°C. in coil (39). After the final turbine (44), wet steam (90% dryness)is condensed in a water-cooled heat exchanger (45) and the condensatepumped away by an extraction pump (46) for pre-heating to 120° C. incoils (47, 48) and returned via the deaerator (49) and recirculatingpump (50) to the steam circuit. Make-up water for the stream circuit isfed in to the deaerator (49).

As mentioned previously, conzone heat recovery also includes the waterheater (40) which provides hot water for the saturator column (30). Fluegases from the conzone (34A) (stream 13 of Table 1) pass from theconzone to atmosphere via a stack (51).

Referring back to the sulphur removal unit (27), the effluent streamcontaining the removed sulphur compounds (stream 8 of Table 1) containsabout 25% (molar) of sulphur compounds and is very suitable fortreatment and sulphur recovery using a Claus kiln (29). The tail gasesfrom the Claus kiln (29) are further treated by e.g. a `Scot` process,for this purpose, a stream of reducing gas (stream 9 of Table 1) istaken before the sulphur removal unit. The final tail gases, containingonly traces of sulphur, are fed to the afterburner (34) with otherresidual emissions but still give rise to a stack gas sulphur level nohigher than 15 parts per million.

    TABLE I       Job No. CPG PATENT Page 1 of 3 MASS BALANCE Client -- Plant -- Date     Issue Location -- 12-Aug-87 1 STREAM NUMBER 1 2 3 4 5 6Sulphur STREAM     NAME Raw Gas Shift Feed Shifted Gas Hydrolysed Removal Feed Sweet Gas     COMPONENTS Mol. Wt. kgmol/h mol % kgmol/h mol % kgmol/h mol % kgmol/h     mol % kgmol/h mol % kgmol/h mol %       Methane 16.043 7.70 0.07 7.70 0.07 7.70 0.05 7.70 0.05 7.65 0.05 7.60     0.05 Carbon Monoxide 28.010 5072.60 46.14 5072.60 46.14 14171.30 9.67     1417.30 9.67 1407.63 9.67 1407.35 10.00 Carbon Dioxide 44.009 995.00     9.05 995.00 9.05 4650.30 31.74 4656.70 31.77 4624.93 31.77 4264.19 30.31     Hydrogen 2.016 4749.40 43.20 4749.40 43.20 8404.70 57.38 8404.70 57.36     8347.35 57.35 8345.68 59.33 Nitrogen (+Ar) 28.010 44.00 0.40 44.00 0.40     44.00 0.30 44.00 0.30 43.70 0.30 43.50 0.31 H2S 34.076 118.70 1.08     118.70 1.08 118.70 0.81 125.15 0.85 124.30 0.85 0.40 .00 COS 60.070 6.60     0.06 6.60 0.06 6.60 0.05 0.15 .00 0.14 .00 0.06 .00 TOTAL - DRY     10994.00 100.00 10994.00 100.00 14649.30 100.00 14655.70 100.00 14555.70     100.00 14068.78 100.00 Water 18.015 14224.00  10994.00  7338.70  7332.30      54.20  40.00 TOTAL - WET  25218.00  21988.00  21988.00  21988.00     14609.90  14108.78        Job No. CPG PATENT Page 2 of 3 MASS BALANCE Client -- Plant -- Date     Issue Location -- 12-Aug-87 1 STREAM NUMBER 7 8 9 10 11 12 STREAM NAME     Saturator Feed Claus Gas Reducing Gas Gas Turbine Fuel Afterburn Gas     Turbine Exhaust COMPONENTS Mol. Wt. kgmol/h mol % kgmol/h mol % kgmol/h     mol % kgmol/h mol % kgmol/h mol % kgmol/h mol %       Methane 16.043 6.50 0.05 0.05 0.01 0.05 0.05 6.50 0.05 1.10 0.05     Carbon Monoxide 28.010 1198.00 10.00 0.28 0.06 9.67 9.67 1198.00 10.00     209.30 10.00 Carbon Dioxide 44.009 3629.70 30.31 360.74 74.08 31.77     31.77 3629.70 30.31 634.50 30.31 4834.50 5.71 Hydrogen 2.016 7104.00     59.33 1.67 0.34 57.35 57.35 7104.00 59.32 1241.70 59.32 Nitrogen (+Ar)     28.010 37.00 0.31 0.20 0.04 0.30 0.30 37.00 0.31 6.50 0.31 66291.00     78.24 H2S 34.076 0.34 .00 123.90 25.45 0.85 0.85 0.34  0.06 .00 COS     60.070 0.05 .00 0.08 0.02 0.01 0.01 0.05  0.01 .00 SO2 64.070     0.39 .00 O2 32.000           13600.50 16.05 TOTAL - DRY  11975.59 100.00     486.92 100.00 100.00 100.00 11975.59 100.00 2093.17 100.00 84726.39     100.00 Water 18.015 34.00  0.00  0.30  1463.10  6.00  8580.00 TOTAL -     WET  12009.50  486.92  100.30  13438.69  2099.17  93306.39        Job No. CPG PATENT Page 3 of 3 MASS BALANCE Client -- Plant -- Date     Issue Location -- 12-Aug-87 1 STREAM NUMBER 13  STREAM NAME Flue Gas     COMPONENTS Mol. Wt. kgmol/h mol % kgmol/h mol % kgmol/h mol % kgmol/h     mol % kgmol/h mol % kgmol/h mol %       Methane 16.043 Carbon Monoxide 28.010 Carbon Dioxide 44.009 5679.00     6.71 Hydrogen 2.016 Nitrogen (+Ar) 28.010 66297.00 78.28 H2S 34.076 COS     60.070 SO2 64.070 0.46 O2 32.000 12720.00 15.02 TOTAL - DRY  84696.46     100.00 Water 18.015 9843.00 TOTAL - WET  94539.46

I claim:
 1. A process for the production of power from a carbonaceousfuel, which comprises partially oxidising the fuel with oxygen or anoxygen-containing gas to yield a gas stream containing carbon monoxideat supra-atmospheric pressure, directly downstream of the oxidising stepquenching said gas stream with water, passing the quenched gas streamthrough a boiler in which the gas stream is cooled, thereby raisingsteam in the boiler, separating water from the cooled gas stream,ensuring that the gas stream is at sufficient temperature to initiate anexothermic shift reaction, and then subjecting the gas stream to acarbon monoxide shift reaction whereby at least some of the carbonmonoxide is converted into carbon dioxide with a consequent evolution ofheat, and, after heating the gas stream with heat evolved by the shiftreaction, expanding the heated gas stream to produce power, and thenusing at least a major portion of the expanded stream as a fuel for agas turbine to produce additional power.
 2. A process as claimed inclaim 1, wherein at least some of the water or steam for said shiftreaction is derived from the quenched water.
 3. A process as claimed inclaim 1, wherein the temperature of the shifted gas stream is increasedprior to expansion.
 4. A process as claimed in claim 1, including asulphur removal step.
 5. A process as claimed in claim 1, whereinessentially all of the carbon monoxide and hydrogen of the expandedstream is used to produce power.